Among polyphenolic compounds, flavonoids, and especially anthocyanins and flavonols, have important implications for red wine quality. The molecular size of polyphenolic compounds affects their relative bitterness and astringency. Monomers are more bitter than astringent, whereas the reverse is true for large molecular weight derivatives. Polymerization and/or condensation with anthocyanins may play a part in the loss of astringency observed during wine ageing. Extracts containing anthocyanins (ACN), and skin tannins (SKIN) and seed tannins (SEED) were prepared from Vitis vinifera var. Cabernet Franc grapes grown in the Loire Valley, and characterized. Phenolic fractions from three Cabernet Franc wines were also isolated and characterized. Bitterness and astringency of ACN, SEED and SKIN as well as the wine extracts were evaluated by time intensity procedures in citric acid solution and in a base white wine. The grape seed and skin extract contained 95-98 pourcentage proanthocyanidins on a weight basis. SEED contained 28.4 pourcentage galloylated units. SKIN contained only 3.8 pourcentage galloylated units but had a much higher average degree of polymerization. SEED and SKIN were equally astringent when tasted at the same concentration in spite of differences in tannin composition. The SEED fraction was slightly bitterer than the SKIN fraction in the citric acid solution, although no difference could be detected between samples in base white wine. Astringency of ACN alone was much lower than either SKIN or SEED. In both media, addition of anthocyanins fraction to SKIN fraction increased astringency. Astringency of the SEED fraction was significantly increased, but only in wine. In wine, addition of ACN to either SEED or SKIN had no effect on bitterness. Seed tannins and skin tannins were equally astringent when tasted at the same concentration in spite of their differences in tannin composition suggesting that the lower percentage of galloylation in the skin may have compensated for the higher molecular weight. We advise to read the entire article.
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